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Development of a low cost processing unit for friction stir welding (LOSTIR)

Final Report Summary - LOSTIR (Development of a low cost processing unit for friction stir welding)

The strategic objectives of the LOSTIR project were:
- to develop a low cost torque/force monitoring device for use with milling machines for friction stir welding (FSW);
- to develop a tool holder capable of operating at friction stir welding temperatures while protecting the monitoring device;
- to develop friction stir weld tools appropriate to milling machine operating constraints;
- to create an opportunity to produce more aluminium products with their increased potential for recycling.

An extensive survey was conducted of milling machines in Europe in regards to their particular characteristics that influence their potential in making good friction stir welds. This exercise was essential as the LOSTIR device is targeted for use on new and existing milling machines. The outcome of the survey highlighted key considerations to be taken into account when designing the LOSTIR device. For instance because milling machines do not provide the ideal conditions for FSW, research and development (R&D) effort was focused on developing a better scientific understanding of the FSW process in order to ensure that making FSW using milling machine produce good welds. Consequently extensive laboratory experiments were conducted to achieve this in conjunction with a large computational simulation effort.

During the FSW process, the material being welded is subjected to extreme mechanical and thermal conditions. Laboratory tests were therefore conducted to simulate these conditions and obtain material properties data that could be used to improve an understanding of the FSW process and also supply information for use in FSW numerical models. Three candidate aluminium alloys were selected for these tests. In particular it is recognised that during FSW the material being welded is subjected to; high strain rates, high temperatures and high strain.

A low cost friction stir weld monitoring unit was designed and a prototype built and tested during the project. Four key elements of the unit were identified; a measuring unit for recording forces and torques during the welding process, a tool holder capable of removing heat from the tool to protect the measuring unit, FSW tools appropriate to milling machine conditions and also software for capturing the required data and visualisation of the important information.

FSW tools were developed capable of producing good quality friction stir welds on commercial milling machines for the materials investigated using the measurement system to provide process monitoring. A tool holder was designed to securely hold the FSW tool during welding and also dissipate as much heat as possible in order to protect the sensitive electronics and sensor equipment mounted in the weld monitoring housing from the severe temperatures experienced by the FSW tool. Cooling fins and a ceramic heat shield were introduced to ensure effective shielding of the electronics. Subsequent tests on the device under extreme loading conditions confirmed that the final design was satisfactory.

A weld monitoring system was developed to accurately measure the vertical and horizontal forces and torque on the tool. The sensor is machined from one piece of high grade stainless steel, heat treated for maximum strength and stability. The data gathered can be directly linked to the acceptance or otherwise of the weld. In addition, the device monitors two user defined temperatures via thermocouples, one attached to the FSW tool, the other acts as a safety cut out to protect the integral telemetry circuit, monitoring the temperature at the interface between the tool holder and the weld monitoring system.

All of the power requirements for the sensor are transmitted using wireless digital telemetry; this also supports two-way communication for calibration and data collection. The information gathered by the LOSTIR device is displayed to the operator in a clear and straightforward manner using a laptop PC running Labview. The instrument panel displays real time numerical values of forces, torque, the temperature adjacent to the system electronics and (if desired) the tool temperature. The system also has the capability to add real-time event markers to allow correlation between process conditions/stages and the recorded data. The display also shows the current captured data values for the weld in progress indicating whether they are within the acceptable range for satisfactory welding.

Towards the end of the project a series of welds were made using the LOSTIR device and these welds were examined and tested for quality and performance. The following evaluations were carried out: visual inspections, non-destructive testing, bend tests, tensile tests and also fatigue tests. In addition the LOSTIR device was rigorously evaluated by performing a range of welds with deliberate imperfections in the welded material or FSW tool. In all cases the performance of the LOSTIR device was confirmed by producing good FSW as well as being able to adequately indicate to the operator when poor quality welds were being produced. This can provide significant quality control benefits to users.

The end user partners in the project, 'Sapa technology' (Sweden) and 'BAE systems' (UK) have both evaluated the LOSTIR device at their respective sites. They have conducted independent investigations in line with their respective businesses and reported these findings back to the LOSTIR consortium. They have identified significant benefits achievable whilst using the LOSTIR device which could have a significant impact on their business. They have been able to establish an increased confidence in quality of welded joints. The benefits identified by the LOSTIR end user partners are expected to be appreciated by a broad industrial sector that have hitherto been cautious of quality issues associated with FSW.